Field of the Invention
The present invention relates to intrinsically safe limited voltage and current output barriers, located in what is known in the industry as an associated apparatus, and which are required to be installed at each electrical interface between an intrinsically safe device and another circuit located within the associated apparatus not deemed intrinsically safe so that the protective components and construction is not damaged in the intrinsically safe device while connected to the associated apparatus having intrinsically safe limited voltage and current output provided by the barrier circuit.
Description of the Related Art
One type of hazardous location is where concentrations of flammable gases, vapors, or dusts occur. Electrical equipment installed in such locations must be specially designed and tested to ensure it does not initiate an explosion, due to arcing contacts or high surface temperature of equipment.
The introduction of electrical apparatuses for signaling or lighting in coal mines was often accompanied by electrically-initiated explosions of flammable gas and dust. Accordingly, standards were developed to identify the features of electrical apparatuses that would prevent electrical initiation of explosions. Several methods of protection are used. One such method is designing electrical devices in a way so that they cannot produce a spark strong enough, or generate heat hot enough, to ignite combustible gasses and/or dusts.
Various strategies exist for safety in electrical installations in these hazardous locations. The simplest strategy is to minimize the amount of electrical equipment installed in a hazardous area, either by keeping the equipment out of the area altogether or by making the area less hazardous by process improvements or ventilation with clean air. Intrinsic safety (inclusive of non-incendive, equipment and wiring methods) is a set of practices for apparatuses designed with low power levels and low stored energy levels. In such apparatuses, insufficient energy is available under the most onerous fault conditions to produce an arc that can ignite the surrounding explosive atmosphere, and insufficient energy is available under the most onerous fault conditions to produce sufficient heat to ignite the surrounding explosive atmosphere. Intrinsic safety is one of several methods that enable a functional device to not be a source of ignition in a surrounding explosive atmosphere. A device termed intrinsically safe is designed to be incapable of producing heat or sparks sufficient to ignite an explosive atmosphere. As used herein, “intrinsically safe output” means the output of the associated apparatus is incapable of delivering a voltage and current greater than the amount of voltage and current permitted by the certification of the intrinsically safe device to be connected to the output of the associated apparatus (i.e., the output of the associated apparatus is incapable of damaging the components and construction of the intrinsically safe device).
Some considerations in designing intrinsically safe electronic devices include: limiting energies to levels not capable of producing an incendive spark in an explosive atmosphere under the most onerous fault conditions, limiting energies to levels not capable of producing the ignition of an explosive atmosphere by generated heat under the most onerous fault conditions, and incorporating the use of proper methods of spacing, construction, and materials. Elimination of spark potential within components is accomplished by limiting to within safe levels, under the most onerous fault conditions, the stored energy in any given circuit and the system as a whole. Generated heat, under certain fault conditions, such as an internal short in a semiconductor device, becomes an issue as the temperature of a component can rise to a level that can ignite some explosive gases and dusts, even in normal use. Various safeguards, such as current and power limiting by resistors and fuses, are therefore employed to ensure safety in those circumstances where a component can reach a temperature that could cause autoignition of a combustible atmosphere. In many highly compact electrical devices used today, printed circuit boards often have component spacing that make it difficult to reduce the possibility of arcing between components, with or without dust or other particulate matter working onto the circuitry. Thus component spacing, siting, and isolation techniques become important to having an intrinsically safe design.
The primary concept behind intrinsic safety is to ensure that only low voltages and currents enter the hazardous area, that no significant energy storage is possible, and that no significant heat generation is possible. A common method for protection is to limit electrical current by using multiple series resistors (assuming that resistors always fail open); and limiting the voltage with multiple Zener devices to ground (assuming diodes always fail shorted). Further, certification standards exist for intrinsic safety designs, which vary by device type, and generally require that a safety barrier, e.g., a galvanic isolation barrier, not exceed approved levels of voltage and current with specific damage to the current and voltage limiting components.
Equipment or instrumentation designed for use in a hazardous area will be designed to operate with low voltage and current, and will be designed without any large lump sum equivalent capacitances or lump sum equivalent inductances that could discharge in an incendive spark. The instrument usually will be connected, using approved wiring or connection methods, to an AC or DC electrically powered device or via one or more safety barriers all of which will be located in a non-hazardous area. The safety barrier ensures that no more than the approved voltage or current connects to the electrical interface port of the intrinsically safe device.
What is needed, and not known in the prior art, is an improved intrinsic safety barrier, typically located within an associated apparatus (like a battery charger, thus making it have an intrinsically safe output) constructed with off-the-shelf, small printed circuit board components and/or low power/current rated discrete components to provide an intrinsically safe limited current and intrinsically safe limited voltage which also has high voltage and high current electrical isolation. This improved intrinsic safety barrier can be used when connecting an AC supplied battery charger, as an associated apparatus that is connected to an intrinsically safe certified device. The improved intrinsic safety barrier prevents damaging of the protective components in the intrinsically certified device while electrically connected to the associated apparatus with intrinsically safe output.